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12 <title>Configuration management</title>
13 <subtitle>revision 7.0</subtitle>
25 <address><email>wakkerma@debian.org</email></address>
39 <address><email>joeyh@debian.org</email></address>
53 Wichert Akkerman and Joey Hess
58 This text is copyright by the authors under the terms of the
59 BSD license, sans advertising clause.
69 Configuration management is quickly becoming a very important issue.
70 Having programs which do cool stuff is great, but we need to store
71 their configuration as well. We see more and more different
72 configuration systems being introduced all the time, which is not very
73 practical. This text introduces a general configuration management
74 system which flexible enough to be used for all kinds of applications.
84 The configuration space
87 All configuration information is stored in what I call the
88 configuration space. This is a database with a special design
89 which resembles the method we look at configuration information.
90 This is done by defining a hierarchy of information. Each package
91 receives its own space in the hierarchy. Each package is free to
92 use a flat space, or divide its space further into
93 sub-hierarchies. If multiple packages share a common purpose they
94 may use a shared toplevel hierarchy, preferably with the same name
95 as a shared (virtual) package name (for example, both
96 <application>mutt</application> and <application>elm</application>
97 can use <literal>mail-reader</literal>,
98 <application>strn</application> and <application>nn</application>
99 could use <literal>news-reader</literal>). This
100 shared tree can also be used as a default, ie a variable
101 <literal>news-reader/nntpserver</literal> can be used by
102 <application>strn</application> if <literal>strn/nntpserver</literal>
106 Each variable in the configuration space has some information
107 associated with it. Most importantly, it has a value. It also may
108 have a set of flags and a set of substitution data.
118 Each variable in the configuration space is associated with some
119 meta-data. The minimum meta-data associated with a variable is:
120 long and short description, type, and default value. The meta-data
121 is essentially static; the protocol described below does not allow it
125 The meta-data exists in a space with similar naming
126 properties to the configuration space described above, and typically
127 one variable in the configuration space will have associated with it
128 metadata with the same name in the meta-data space. However, this need
129 not be the case; many different variables can all be associated with
130 the same meta-data. In effect the meta-data serves as a template
131 for the configuration variable.
138 So, what do we need to store in a variable template? Of course we
139 need a name to identify the template. Template names are made up of
140 components separated by the character `/' (slash).
141 Each component is limited to alphanumerics and `+' `-' `.'
142 (plus, minus, full stop).
145 A type is also needed so data can be verified. Here is a table
146 of common types; implementations are free to make up more.
150 Of course a default value is useful as well, and
151 finally we need a description of the variable. We actually use two
152 descriptions: a short one (limited to 50 characters or so) and an
156 The extended description may be word-wrapped by the
157 FrontEnd. To make separate paragraphs in it, use <literal>.</literal>
158 on a line by itself to separate them. Text in the extended
159 description that is prefaced by additional whitespace will not be
160 wordwrapped. Both the description and extended
161 description may have substitutions embedded in them. Ie,
162 <literal>${foo}</literal>. These will be expanded when the
163 descriptions are displayed.
166 This information is stored in a template file that consists of
167 stanzas in a rfc-822 compliant format, separated by blank lines.
173 Description: unqualified hostname for this computer
174 This is the name by which this computer will be known on the network. It
175 has to be a unique name in your domain.
179 Description: domain for this computer
180 This is the domain your computer is a member of. Typically it is
181 something like "mycompany.com" or "myuniversity.edu".
188 Configuration frontends
191 Of course applications can use the database and meta-database directly.
192 But there should be a simple system to interact with the user that is
193 simple and modular enough to be used with systems ranging from
194 shell-scripts to Fortran programs. To do this we define a general
195 frontend that can be driven using the simplest and most common form of
196 communication: stdin and stdout.
199 Using this simple form of communication gives us a great advantage: it
200 becomes easy to change the frontend. That means the user can switch
201 between a console, a graphical or even a web-interface at will.
204 Besides being able to switch between types of frontends there is
205 another important aspect of a good user interface: user friendliness.
206 We have to account for the fact that some users know more then others
207 and change the information we show or ask from the user. We do this by
208 giving everything a priority and giving the user control over what
209 kind of questions he wants to see. Experts can request to see
210 everything, while novices get the option of only seeing only important
211 questions. Finally there is an option to simply skip all questions, so
212 it becomes possible to do automatic configuration using default values
213 or values that are downloaded into the database from a remote
214 location. This makes it simple for example to install and manage
215 clusters or lab rooms or do installs for dummies.
220 Communication with the frontend
223 This communication between the frontend and the application should be
224 as simple as possible. Since most IO implementations default to
225 line-buffered IO, so we use a simple language where each command is
229 After sending each command to stdout, the client
230 should read one line from stdin. This is the response to the command,
231 and it will be in the form of a number followed by whitespace and an
232 optional string of text. The number is the status code, while the
233 text provides additional information.
237 Here are the currently supported commands.
245 Debian install-time configuration
248 Debian has had an excellent packaging system for a long time now. There is
249 one thing missing though: a system to handle the configuration of
250 packages so we don't have to stop the installation every time a package
251 needs some data from the user or wants to show some information.
254 We want to make a package which does not break older dpkg's, and we
255 want to be able to get the configuration information before the package
256 is unpacked. To do this we add two new files, config and templates, to
257 the control.tar.gz of a .deb package. Since all installation-software
258 (apt, dselect, dpkg) download the package before installing it, we can
259 extract this before the package is unpacked.
262 The templates file lists the templates for variables that this package
263 uses. This is done using the format as used in the example in the
264 section on templates.
267 The config-file contains a new element, which I call the configmodule.
268 This is a program that will determine the configuration before the
269 package is unpacked. This means it is run <emphasis>before</emphasis>
270 the preinst, and before the package is unpacked! This is done to make
271 sure that we can use the desired configuration in the preinst if
275 How does the configmodule get its information? The configmodule
276 needs a way to retrieve information from the configuration space, ask
277 the user for information if necessary, etc. But we don't want to
278 implement a user interface for each package. To solve this we use a
279 separate frontend as specified in the section on frontends.